Telephone signaling apparatus



Dec. 21, 1948. B. J. KUCERA TELEPHONE SIGNALING APPARATUS Filed July 15, 1942 FIG. I

7 Sheets-Sheet 1 TRANSIENT BETEGTOR 4o OPERATOR'S HEAD SET 25 INVENTOR. BARNEY J. KUCERA ATTORNEYS,

Dec. 21, 1948. B, J KUCERA 2,457,046

TELEPHONE SIGNALING APPARATUS Filed July 15. 1942 '7 Sheet-Sheet 2 hay b 1%. SIGNAL CURRENT m 746 1 REPEATER 30 74b 14 E-W I I w-E AMPLIFIER AMPUHER T IOM *4:

IE\\-"ENTOR. BARNEY J. KUCERA B a .[f I I g I mommy:

Dec. 21, 1948. B, J, KUCERA 2,457,046

TELEPHONE S IGNALING APPARATUS Filed July 15, 1942 7 Sheets-Sheet 3 IM PEDANCE DETECTOR 60 SIGNAL CURRENT REPEATER 30 BAL. NET.

INVENTOR. BARNEY J. KUCERA BY m g (441 M Dec. 21, 1948. Y B. J. KUCERA v 2,457,046

TELEPHONE SIGNALING APPARATUS Filed July 15, 1942. r 7 Sheets-Sheet 5 I F 5 TRANSIENT DQEGTOR 14o P A SE S EQ FFLIZGR T I24 35 v v v v nnnmo INVENTOR. BARNEY J. KUCERA ATTORNEYS Dec. 21, 1948. B, J, KUCERA 2,457,046

, TELEPHONE SIGNALING APPARATUS '7 sheets-sheet, s I

Filed July 15, 1942 2 l7 DlAL TONE CURRENT souaceo E"'W c a W E AMPLIFIER AMPLIFIER I0 H U 7 F U I I INVENTOR.

BARNEY .1. KUCERA Y I B M ATTORNEY 1948- B. J. KUCERA- 2,457,046

TELEPHONE SIGNALING APPARATUS v Filed July 15, 1942 Y '1 Sheets-Sheet 7 FIG. 7.

IMPEDANCE, DETECTOR I60 SIGNAL CURRENT REPEATER I30 INVENTOR. BARNEY J. KUCERA WM,M M

ATTORNEYS iatentecl Dec. 21 1948 TELEPHONE SIGNALING APPARATUS Barney J. Kucera, Cleveland, Ohio, assignon to Automatic Electric.Laboratories, 1nc., aacorpoiration: of Delaware Application July 15, 1942, S'etialNoA503987 23 Claims. 1

The present invention. relates. to signaling apparatus and, more specifically, to. improvements in apparatus for secretly observing all calls handled at any selected substation of an automatic. telephone system.

In the usual wire tapping or call observing arrangement, jumper connectionsare made to the conductors of the line extending to the substation which is to be observed; such: jumper: connections maybe made either at the line terminals'of the mainframe in the terminating oifice, at some point along. the. line extending tothe substation under observation, or on the premises where the substation under observation is located. This arrangement has the disadvantage that the equipment must be. moved from placeto place and the wiring connections must be changed, in order operatively to associate the observing equipment with different substations of: the system.

It is an object of the present invention, there'- fore, toprovide'improved' apparatus of the char-- acter described, which is so arranged; that. any desired substation of an automatic system maybe brought under observation from aparticular station or point of observation, without changring the wiringioi: the-apparatus and withoutrunhing jumper connections to. the conductors of choline extending to the substation. which itvi's' desired to observe.

According to another object of the invention, the subscriber controlled automatic: switching apparatus of the. system; isused forselectively associating: the observing equipment: with any desired substation of the system;v

In accordance; with a further object of the invention, the subscribercontrolled automaticswitching apparatus of the system is utilized to; prevent the substation. under observationv from receiving incoming calls and. to force all'v calls originating at the substation under observation to be first routed. to the observationipoint, and facilities are provided at this. pointfor routing the calls totheirxdesired destinations;

According to still another object. of theinvene tion facilities are provided at the point of oblsalvation for transmitting to the substation under observation appropriate signals. as. to the "progressof a call initiated at this:substation.v

In the illustrated embodiments of the invert.- tion, the observing equipment is. operatively' associated with a desired substation by first rouiiing a connection from the point of observation through the automatic switching apparatus of the system to the desired substation. Thecone nection is then held from the observation point after it. isreleased. atthe. substation and, as finally completediinclu'des a reactive talking bridge-"atv the final switching stage thereof. This talking bridge: has the effect of preventing dial impulses from: being transmitted: from the substation tothe: point of. observation. More specifically, with the'brid'ge included in the connection',v operations oi /the substation. calling device and of thehoolt switch provided at. the substation. under. observation manifest. themeslvesas electrical voltage: transients; at the observed end of the connection. Moreover; the opening and closing of the signaling circuit. at thesubstation end-thereof, incident: to: the. initiation of a call at the substation, causes corresponding changes in the" impedance of the; signaling; circuit as measured at theobservedendthereof.

It is another object of the invention to provide in: a switching and signaling. system of the character described. an. improved arrangement for utilizing.the'transientsl produced on the signaling circuitifor eftecting the extension of a connectioni from the: signaling circuit. to another line.v

According; to-ranother object of the invention, facilities con-trolled: by certain of the transients appearing ornthev signaling circuit are provided for transmitting, a. signal: of limited duration back over this circuit.

According to still. another object of the invention. those. facilities which are provided in the" apparatuslto. respond to transients appearing. on the-signaling circuit, are.- arranged in an improved. manner to discriminate between a transient produced-byopening. the signaling circuit and a transient producedby closing of the signaling. circuit.

It is. another object of the invention to provide in a signaling and switching. arrangement of the character described, improved apparatus for transmitting impulses to automatic switching equipment,,-which apparatus is arranged to be controlled.- by the. changes in impedance of the signaling. circuit, as. measured at one end thereof, which result. from. the opening and I closing; ofitthe circuitat the other end thereof.

According to-still another objectof the. invention; provisions, are. made in the. apparatus for preventing the impulse transmitting equipment, andthatapparatuswhich responds to the transients-v appearing. on. the signaling. circuit, from falsely respondingtothe: signal and. transient voltagesawhiclr areimpressedon the circuit incident to 'thewoperation -of the automate switch ing equipment to: extend a connection from the signaling: circuit-i The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figs. 1, 2 and 3, when laid side by side in the order named and combined with Fig. 4, illustrate an improved system characterized by the features of the invention briefly outlined above; and Figs. 5, 6 and 7, when laid side by side in the order named and combined withFig. 4, illustrate a modified arrangement of the system, also characterized by the features of the present invention.

Referring now more particularly to Figs. 1 to 4, inclusive, of the drawings, the improved observing apparatus is illustrated in its circuit relationship to the lines and automatic switching equipment of a thousap d line automatic telephone system. More specifically, the call observing equipment, as provided at a particular point of observation, and illustrated in Figs. 1, 2 and 3 of thedrawings, is'connected to the automatic switching apparatus of' the system as schematically shown in Fig. 4 of the drawings, by means of two lines It! and ILwhich respectively terminate in the exchange of the system in line switches I 4 and I5. Two additional subscriber lines 8 and 9 are shown in'the drawings as extending to the substations A and B, respectively.

These lines also terminate in the exchange in line switches I2 and I3, respectively, through which access is'gained to the impulse-controlled numerical switches of the exchange. More specifically, the impulse-controlled switching equipment of the exchange comprises a plurality of selectors It, I'I, etc., which are accessible to the line switches individual to the various lines and have access, in turn, to ten groups of connectors, two

of whichrespectively include the Connectors [8 and I9. Each group of connectors has access, in turn, to a particular group of one hundred lines. It will be understood that the line switches I2, I3, M and I5 and the other similar switches of the system are non-numerical'switches' in the sense that they are ineffective to perform any line selectingfunction other than that of selecting an idle one of the selector switches. The

selector and connector switches, 'on the other M hand, are commonly known as numerical switches in that they are operative to perform particular line selecting functions in response to impulses transmitted thereto. Each ofthe line switches I2, I3, I4, I5, etc., is preferably of the well known rotary type, of whichthere'are several commer- 1 cial embodiments. Each of theselector and connector switches is preferably of the well known Strowger type having embodiedtherein a wiper carriage structure which is adapted to be translated vertically to bring the wipers thereof to a position opposite a particular level of bank contacts, and to then be rotated to bring the wipers into engagement with a particular set of bank contacts in a selected level; vertical, rotary and release magnets; and controlrelays suitably connected and arranged to control the energization of the various magnets and the line switching in accordance with the particular functions of the particular switch.

In order more easily to explainthe operation 1 of the circuits to which the present invention pertains, a portion of the equipment embodied'in the connector I9 has been shown in detail. This connector is'p'rovided with a reactive talking bridge which, in' the illustrated arrangement,

comprises the two condensers 28a and 28b.

air/pie These two condensers have the function of separating the calling and answering loop circuits included in a connection set up through the connector IS in so far as direct currents are concerned, while at the same time providing a signal current channel through which signal currents may be transmitted between the two loop circuits. The connector I9 further conventionally comprises a line relay 2'! having its windings bridged in series with the exchange battery across the talking conductors of the trunk incoming to the connector, and a back-bridge relay 29 having its windings adapted to be bridged in series with the exchange battery across the talking wipers of the connector. As thus connected, the windings of the two relays 2'! and 29 are respectively included in the calling and answering loop circuits of a connection completed through the connector I9 between two lines of the system.

The substations served by the system are of the well-known common battery type. Only a portion of this equipment as provided at the illustrated substations A and B is shown in the drawings. In brief, this equipment, as provided at'each of the two substations, comprises a trans mitter H, a receiver 22, a calling device 20 of the dial-operated type, and a hook or cradle switch 23. It will be understood that each of the substations also includes a ringer bridged across the conductors of the associated line in series with a condenser, and an induction coil suitably con-' nected in the circuit to provide the well known anti-side tone type of voice current transmission. In the interests of simplifying the disclosure, these elements have been omitted from the drawings.

In general, the call observing equipment as shown in Figs. 1, 2 and 3 of the drawings comprises a calling device 24 through operation of which calls ma be routed from the point of observation through the automatic switching equipment to any desired substation of the system', an operators headset 25 which conventionally includes a transmitter and receiver, and a dial and talk key 25 Which may be used selectively to connect the calling device 2 5 and the headset 25 to the line I (I. The equipment further comprises a signal current repeater 38 which may be connected between the two lines I0 and I I through operation of the keys 3'! and 38 and is operative to transmit signal currents between these two lines in either direction. Briefly considered, this repeater comprises terminal hybrid systems which respectively include the two hybrid coils 3| and 33 and two associated balancing networks 32 and 34. These hybrid systems are connected by an east-west signal current transmission channel which includes the schematically illustrated vacuum tube amplifier 35, and by a west-east signal current transmission channel which includes the schematically illustrated vacuum tube amplifier 36. It will be understood .in this regard that the amplifier 35 as provided lsient. detector-AG, it will he noted that this vdetector type, such that a-it =wil1 operate and vremain oper- -:ated when either of its -"two windings .is ener- :gized but will restore when 'both of titsrvvindings are concurrently s energized. Finally, the .itWO

relays 11 and -S'l arezprovided Withtboth operating :and holding windings. More specifically, Tithe ilower winding of eac'hmf' these relaysmaybe-prei energized :without causing z'the :relay to-coperate.

':When,1:.however, the. upper :opera'ting iwindingioi either rof :these. relayseis.;subsequently:aenergized lto cause'the operationi thereof, normalrrateda'current flow 7 through :the i lower winding of the relay .vvilli-serve to .hold the relay in sits operated position :after .the upper twindingof the ,relay is :de- M energized. A keyiiM -is provided/yin the equip- :ment to control the ipreenergization of the hold- ,ingawindingszfor the ltwo relays iizliand :81 and to' provide :a .locking circuit for; the relay 89.

As will he ,apparent .frorrrthe following detailed. consideration 50f the disclosed-circuits, the

operation of the apparatus requires that the relay 119 :be released 1a predetermined time interva'lgafter itis winitiallyioperate'd. For the purcurrent limiting rheostat "96. v For the :purpose .of disassociating the-illustrated vcontrol equipment froma connection ;setup through the .point of observation, :after the functions ,assigned to this equipment have been discharged therebyia ekey 1450f the vWell "known locking type and having two operating positions-is provided.

Referring "now more particularly to the tranis .arranged tocontrol the relay l-and that-its input terminals are coupled "to the. output terminals of the channel amplifier. 35 by means of the coupling condenser -48. This detector comprises a three-electrode transient amplifier tubes of the indirectly heated cathode type; a three-elec- .trode control-i tube 42,.a1sO-0f. theindirectly heated catfmtidetype; and a full Wave copperyoxiderectifier 43 having. its input terminals -coupl-ed. to the outputrelectrodesloi the tube 42 through a coupling. transformer A l and its .output terminals .bridged by a condenser liihaving the function of controlling the operate and releaseperi-ods ofthe :relay is. More specifically considered, the input electrodes dlg andMciare .coupledto the output terminals of the amplifier '35 through. 1a,=resis't ance-capacitance coupling network .Whichincludes theresistor .41 andthevcondenser-AB. A cathode biasingcircuit ifiicomprising the shunt-connected resistor dfiaand condenser Aficis includedin the output circuit of the tube AI -to provide the required ncrmalbias voltage betweenthe input electrodes attic and ligefvtheztube-r l-l. .Thevoutput aeleotrodes1Haoanda lic oftheamplifier tub-er-kI 1 are icoupled to the inputielectrodesMgandMc of the control tuhe i lfl through 2 an resistance -ca'pac i-tance netwo'rk vvhichincludes the resistors-5 hand 52=arrd the condenser d'ii. The normal-' bias -loetween the input electrodes of the tube 4'2 isprovided -by a 'C '5 ha-ttery'ififi which is nonnected between these-electrodes' in'series With-"the two resistors "52 and-"52. This-bias voltage -rnay be increasedunder-the control of the relay ii l 'by- -shun-ting theresistor 5! lay an -additiona1 C battery SS haVingits I terminals so poled that the voltage thereof is added-to the :voltage of the battery hfi. Anodevoltageforthe "two 'tubcs d I aud t? issuppliedby a==current souroe 54 rha-ving its positive terminal connected-to the anode o lza 'through the primary wvin'dingof the coupling transformer- 44 and to the anode Ma ithroughxt-he inductance elemen-tdit.

Thegimpedan-ce de'tector fill, as shown in the -up- .per ,por.tion:oi Fig. 3-:0f the drawin-gs, comprises 1.-a,thermionic audio irequency oscillatoremploying .thesfivrz-electrode oscillator-tube 55i a rectifier tWhl'Ch includes -ithe three-electrotl-e 'vacuum 'tuhe -E;B,. and pulsing; relay 82 having its Windingdneludedin theoutput "circuit of the rectifi-er tube 158. More specifically, the oscillator-section or the :deteetorefid-comprises-a condenser 83 and=the=sec- :ondary windinlgfli ibsof a "coupling transforrner td, which are shunt connected toform a frequency ideterminingttank circuit. This tankcircuit is'oou n-led to .theigridzt in and cathode t i c of: the tube t i through itheyibyapass vcondenser 8%. 'I 'he -screen electrodezfi lgi acts :the: control grid of theoscillator; and a portion of the oscillatory voltage de- .velopedzacross:thetuned: tank circuit-is impressed eon thisz-grid' through the cohdenser'ififi andjthe intar-electrode:acapacitance between this electrode qanlchthezgridafiip. The potentialodifference between (the eontrolsgridiig' and lti'l-e cathode fiic ofith-eatuhe, i. estheibiasvon the grid it! o, isd'eter- :mined :by :the :resistor :13 connected. between thesetwo elements. illhe output electrodes'of'tho OSCUIBIDITIl-lbfilfil gareucoupled rto the input eleci trodes ofthe reotifierztube 6:8 byt- -means o'f-a coupling transformersfil. More particularly, the electronstream within z'the' tube 2E! and (specifically zthatvrhioh:extendsto the anode 51a, as .modulated :by;changesinltheipotentialionrthe gridfi #51,.15 used to couple theyprimaryiwindingiof=the transformer .61 with the oscillatory portion .of the tube: circuit. Elfectivelmthe arrangement just described is atuned :plate.icircuit:ns-cill-ator, with thei grid -16 E 39 actingvasxthe:anode and the'screen electrode :6 lo acting-as thecon'trol gridgtoj indirectlyproduce change-sin the electr-onistreamzin response .tospo- -tential changes. thereon. This; arrangement, While "55-'capable of producing therequisite output voltage i across theprirnary .-\vinding. of theeoupling transformer 6?,is not capableofihandlirig-any substantialrload. Accordingly; whenthe impedance across its input terminals 89' isaltered to effectively increase the load imposed thereon, the oscillator immediatelyoeasesrto operate.

Anode potential for-the two tub-es fii "and-taxis supplied-byadirect current source E2, the negative terminal. of whichiiswconnected to the cathodesof thetWo-tubes. Th-epositiveterminal-of this source isiconnected to the anode of thertube 158 through the windingof the relay QZ andto the anode E-ia through the primary Winding of the transformer "61. ,It is'also connectedto-theelectrode B-ip-of'the tube 5! through the secondary Winding of the coupling transformer 64. The rectifier tuloefifl is normally biased slightly beyond :cut-ofi 'hy the voltage of a C battery :71 l, which voltageis' negatively applied to the :controlxgrid of the tube 63 75 through theisecondary awinding of the coupling transformer 61. Current for energizing the respective relays 11, 19, 8|, 85, 81 and 89 may be derived from any suitable direct current source. The positive terminal of this source is connected to 'all terminals indicated by the ground symbol while the negative terminal thereof is connected to those terminals identified by the negative polarity sign. This source may also be used to energize the respective cathode heaters of the four tubes ll, 42, 6| and 68.

In considering the operation of the equipment thus far described, it may be assumed that an attendant at the point of observation desires to monitor all calls originating at the substation A.

In order to condition the observing equipment for operation, the key 14 is operated to the position illustrated in the drawings, and the key 94 is actuated to its closed-circuit position to complete obvious circuits for energizing the respective number assigned to this substation. When the,

key 26 is thrown to its dial position, a loop circuit including the conductors of the line ID, the contacts 26a and 26b, and the pulsing springs of the calling device 24 is completed for energizing the line relay conventionally embodied in the line switch [4. In response to this operation, the line switch l4 functions in a well known manner to select an idle one of the selectors to which it has access Assuming that the selector I! is seized by the line switch [4, the loop circuit is extended to.

this selector to cause the line and hold relays embodied therein to operate and complete the usual dial tone signaling circuit, whereby a dial tone signal is produced by the receiver of the operators headset 25. When this signal is received,-

the attendant may actuate the calling device 24 in accordance with the three digits of the directory number assigned to the desired substation A. Incident to the off-normal movement of the dial embodied in this device, the lower shunt springs thereof are closed to complete an obvious path for short-circuiting the operators headset 25 in order to lower the resistance of the dialing circuit and to prevent the opening and closing of this circuit from producing clicks in the receiver of the headset. These springs are opened to interrupt the short-circuiting path each time the dial of the calling device is returned tonormal. During each return movement of the dial, the upper pulsing springs of the calling device are opened and closed a number of times, determined by the numerical value of the digit dialed. In this regard it will be understood that the impulses of the first digit are utilized to elevate the wipers of the selector I! to a position opposite the level of bank contacts through which access is gained to the connectors having access to the line 8. During the inter-digit pause separating the first and second digits, these wipers are automatically rotated over the contacts of the selected level successively to test the connectors of the selected group until an idle connector is found therein. Assuming, for example, that the connector I9 is the first available connector of the group, operation of the selector I1 is arrested and the calling loop circuit is switched through to this connector when the Y wipers of the selector are positioned on the bank contacts terminating the conductors of the trunk incoming to this connector. Incident to this operation the windings of the line relay 2'! are included in the calling p circuit. After the connection has thus been extended to the connector I9 and when the final digits of the directory number assigned to the substation A are dialed, the wipers of this connector are positioned on the contacts terminating the conductors of the line 8. More specifically, the second digit as dialed into the connector I9 is utilized to elevate the wipers of this connector to a position opposite the level of bank contacts in which the conductors of the line 8 are terminated, while the third and final digit is used to rotate the connector wipers until they engage the particuluar bank contacts to which the conductors of the line 8 are connected. After the line 8 is thus selected it is tested in the connector I 9 to determine the idle or busy condition thereof, and if busy the usual busy signal is transmitted over the calling loop circuit to energize the receiver of the operators headset 25. If such a signal is received the attendant at the observation point may release the connection by restoring the key 26 to its normal position to open the calling loop circuit. If the called line 8 is idle at the time it is selected by the connector [9, the control equipment of this connector operates in an entirely conventional manner to transmit ringing current thereover for th purpose of energizing the ringer provided at the substation A. Incident to this operation, ground potential is applied to the private wiper of the connector for the purpose of marking the line 8 as busy to the other connectors and thus guarding this line against incoming calls from the other substations of the system. The desired conversational circuit between the observation point and the substation A is completed when the receiver 22a is removed from the supportinghook of the hook switch 23a. More specifically, when the contacts of this hook switch are closed, an answering loop circuit is completed which includes the winding of the ring cut-01f relay conventionally embodied in the connector I 9, the conductors of the line 8, the pulsing springs of the calling device 200., the transmitter 21a, the receiver 22a and the contact springs of the hook switch 23a. In operating, the ring outoff relay of the connector 19 operates to terminate the transmission of ringing current over the line 8 and to extend the answering loop circuit to include the windings of the back-bridge relay 29. Following these relay operations in the connector I9 the desired talking circuit is completed.

It will be understood that the attendant at the observation point must have a reasonable pretext for calling the substation A in order to prevent the answering party at this substation from suspecting the nature of the call. At the conclusion of the conversation, the attendant maintains the calling loop circuit closed in order to prevent the release of the operated switch train through which the connection extends. In this regard it is pointed out that the connector I9 is either of the last-party release or of the calling-party release type. With either type of connector the only operation resulting from the opening of the answering loop circuit at the substation A is that of deenergizing the back-bridge relay 29. Thus, following the release of the connection at the substation A, the line 8 extending thereto remains guarded against incoming calls and all calls initiated at this substation can only be extendediover aroutexwhich includesathe com.- pleted connection to the. point. of observation.

the conductors of the lines I0. and to. the..ter-

minals of the hybridcoilsi Landfill, respectively, and operatively to. associate the. control equip ment with theselines. After. theikey ifllis oper:

ated to its closed-circuit position, two parallel.

direct current paths are bridged across the conductors of the line it, one of these pathsv including the contacts 'Ma and Hi and the series-connected windings of the hybrid coil 3.1, and the other including the contacts Maand the primary Winding Giant the coupling transformer 64. It will be appreciated that after these paths are closed, the dial and talk key 26 may be restored. to normal without releasing the connectionas set through the switches 14, I! and IE to. the line 8.:

At this point, it is in. order to comment briefly on the factors which are utilized in the control apparatus at the observation point to effect the extension of a connection to the desired substation. Briefly, these factors are the change in impedance produced across the input terminals 59 of. the impedance detector 6.9 incident to theopening and closing of the answering loop circuit extending between the connector i9 and. the substation A, and thetransient voltages which are. produced across the conductors of theline In in response to the opening andclosing of the an.- swering loop circuit. In this regard itv will be understoodv that when the contacts of thehook switch 23a for the pulsing contacts of the calling device 26a are closed to complete the answering loop circuit, a. voltage transient is produced across the conductors of .the linev l8 and appears across the line terminals of the hybrid coil 3|, the terminals 69, and the terminals of the re.-. ceiver in the operatorsheadset 25. This transient manifests itself as a click in the receiver of the. operators headset. Also incident to the closing of the answering loop circuit, the impedance of the overall signalingcircuit as measured at the line terminals of the hybrid. coil 3i and. the inputterminals, 69 .ofthe. impedanceadetector; 'ziiiis substantially changed, assuming that the lines ii-andilEl arenotunduly-long. Mores-pecifir cally, this signaling circuit effectively comprises two links which arerather closely coupled through the condensers 28a and 28bv of the connector talking bridge. these links,- i. e., that comprising-the answering loop-circuit; is effectively short-circuited through the substation equipment at the substation A, the impedance of the circuit as measured at the far. end of the other link, i. e., across the line terminals of the hybrid'coil 3| and .theterminals. 65, is substantially decreased. Each time the answering loop circuit as completed. between the; connector i and the substation A is Opened, either atrthe-contact-springs offithe hook switch 23a, or at the pulsing contacts of the calling: device 26a; a voltage transient is produced across the conductorsofthe line' Ills whichiissubstantially' greater in amplitude than: that: produced When; the answeringloop circuit is; closed. In this case the counter electromotiveq-iorce-developed in the windings of 'theback-bridge-relay 29 additively combined with. the. voltage of the exchange: battery: and impressedthrougn the condensers" 28a; and128b: across the conductorsrof.

Accordingly, when oneof. 1

, relay. 1.1T closest:

theeline; III-and: hence-.acrosst-the lterminalstofi-ther hybrid coil 3;! v and? the; input terminals ,69 of; the

impedance detector 60.. A150,;whentheanswere ing: loop circuittistopened ;the impedance; :of; this; loop. circuit." as reflecteditintmthescalling loop cir,- cuiiris substantiallyincreased, whereby-the-z-efiece tive. impedance-tot, the..o.verall signaling.- circuit as? measured-latsthetline; terminals.- ofwthe hybrid coil 31 'andetheainput terminalszfifil ofsthei detector. 63 is lllC-ICEQ-Sfidtl. sos-longgjasszthalinee windings: of the hybrid coils-.31 are: connectedtin-ishunt. with;

the; terminals: Gas-through .thewontacts 9 the;

low impedance: of theseawindings, sufiiciently loads the oscillator section .ofithecdetectorfifll tov prevent: oscillation;thereot.-- Whem however the contactsssll areqop enedsto disconnect -.theser.wind::- ings" from across the terminalatfl ,the operation; of tthe; oscillator; circuit .r is,-.-.controlled; iineaccorde ance withcthe-loading .aifordedsby the impedance,

of 4 the-:connection as measured .at. the terminals: 6.9: Thus thetroscill-atou portion of the; {impede ance-deteetonfit) ,issonly'lighfly; loadedl-andr-hence. rendered-capable oftoscillating durmg thoseepee rio ds when; the: answering lqop circuit..,-extending to; they-substationeAs is op ened but is rendered; incapable; of; oscillating; immcdiatel-y thfir. loop;- circuit.;connectinggthefsubstatiomAswith the cone nectoir L9; isrrreelosed1;duei: ti0.-;thea increased. load, imposedrthereom... t p I From the above explanationz-it -.,wi-l1 -be 'undere stood that when, as c-alliis-isuhsequently i iti ted at thee SHbStatiQHRAsx or transient 'volta esapnears betweez-lrthez conductors .of theelinm l 11a whichszis; impressed-upon. ithesline terminals OilbhQrhYbIF-idk coil,- 3 l-:,. This; transientt srttransmitteda-lthrough the. windings oi; theahtrbridscoih :3 It, amplified hy the zchannelramplifier. isandimpressed lupm'l-rthe: input: electrodes tot eramolifieritubes 44: through the condenserrd ThBitI'flXJ-Sifillbi voltages is furs ther; amplified "-througqh-thatubez dill-and impress ed acrossthe input:.electrodesiofuthe- .control-ztuba lh through:theacondenserr 49:. The .cresulting (T131311? sient; current fiow in. the. output l cine-nitrofiatho tube diz-causesa an-iinduced.alternating voltages of two oppositely poled: halt-cycles toybet. developed intthe-isecondary winding :of; :the:.-,couplinggtransrformer: M-whichds impressed-iuponlthe.iinputztare minals-lofathesfull.a avesrectifiem w. This-svelte,- age--is.--;rectified tnesrec-tifieFL-AB -,-and .impressed across the winding of the relay l5 throughithe.

contactstfi.

Whenlthus: momentahlygcnerniMd; therelayfli c-loses. gitsscontactssl 6 .atol complete ansohvio-usrcire cuit:.-for. ene-rgizinggtheirelawfl-li-. In ioperatineitthe.

it u contacts..-.l8 to completean obvious-i circuit. for; energizing-thee parallels-con,- nectedl-windingseoflthe threerrela'ysr' 19-, 8'l.-and;&5 It" is? noted. that iollowingg the i-QDS rationc-of; the relay; IL-.the current traversing the? lower. orholdin swinding of. thisrelay -isisufficientrtoimain-w tain the rrelay itssxoperated ositionyafterr the. relay-I151 releasesito :de'ener ize ts-iupperiwinding; Itmay bernotedirfurther thatftheicondenson -.-.as bridged a-crossttheeoutnutmerminals of; the--zr,ecti= fier, 43:15 utilizedato sustain; thezenergization; or the-relam15aunti1 thawfou-n-relays time; Bill-and: 8Shave-completedrtheirioneration.

The 1 relay *1 9 upon 1 ope-rating; opens: its con: tacts. .801 todeenergizeathe relay 15:..andathuss-prer vent theslatterre lay;'from1 .-r,esponding :,to ifurther transients voltages. appearing, across;;- the, con: doctor-s: of thealinor until: .suchz. time as the relay: This; deenergizedr. The irelay warlun p l:- atingg 0105.652: .its'iscontactsl- 82: to; connect ,the C batteryififlnzshuntlwith.themesistor 5.1;: andltheree by increase the negative bias on the control grid 42g of the tube 42 to a value substantially beyond the point of anode current cut-E. Thus the transient detector 40 is prevented from passing transient voltages of small amplitude that may appear across the conductors of the line I 0. More specifically, the magnitude of the bias voltage impressed between the input electrodes of the tube 42 is, following operation of the relay 8|, sufiicient to prevent the transient detector from transmitting any transient voltage except one equal to or greater than that produced by opening the loop circuit which connects the connector l9 with the substation A.

In operating, the relay 8| also closes its contacts 34 to complete a loop circuit between the point of observation and the line switch I 5. This circuit includes the conductors of the line H; the contacts of the key 38, the series-connected line windings of the hybrid coil 33, and the contacts 93, 84 and 140. In operating, the relay 8| also closes its contacts 83 to prepare a circuit for energizin the parallel-connected windings of the relays 8! and 89. It is noted in this regard that the relay 8| is so adjusted that the contacts 83 thereof are the last to close and are only engaged after the relay [5 has released to open its contacts 16. Thus the relays 81 and B9 are prevented from operating in response to the initial operation of the relay 15. a

When the above-described loop circuit is completed by way of the line H to the line switch l5, this line switch operates to extend the circuit to the first idle selector to which it has access. Assuming that the selector I 6 is seized through the line switch l5, the callingloop circuit is extended thereto and the control equipment of this selector is operated to complete the usual dial tone signaling circuit. The signal voltage thus impressed across the conductors of the line H is transmitted through the hybrid coil 33, amplified by the west-east channel amplifier 36, and transmitted through the hybrid coil 3| to be impressed upon the conductors of the line I0. From thisv point the signal current is transmitted over the connection established between the observing point and the substation A to energize the receiver 22a and thus inform the calling party at this substation that the dialing operation may be started.

Incident to the extension of the connection from the observation point through the line switch IE to the selector l6, at least two line switching operations are performed in these two switches, which result in transient voltages appearing across the conductors of the line H. Such voltages together with the dial tone signal voltage are transmitted through the hybrid coil 33 and the amplifier 36 to the hybrid coil 3|. If the impedance of the balancing network 32 exactly matches the line impedance as measured .betweenthe line terminals of the hybrid coil 3 1, these voltages are not impressed upon the input terminals of the amplifier 35. In practice, however'fia" precise matching of the two indicated "'impedances'fi s seldom if ever obtained, with the result that a certain amount of unbalance of the hybrid system, representinga certain amount of coupling between the west-east channel and the east-west channel, obtains. Accordingly, portions of the above-noted transient and signal voltages are transmitted through the hybrid coil 3 I, amplified by the amplifier 35 and impressed upon the input electrodes of the transient detector 40. Since,

however, the relay 19 is operated during the interval required to extend the connection from the observation point over the line H to the selector IS in the manner just explained, it will be understood that if the transient voltages produced incident to the extension of the connection and transmitted to the input side of the transient detector 40 are passed through this detector, the relay 15 is prevented from operating. Thus, so long as the relay 79 is operated the operating circuitfor the relay is held open at the contacts 80.

After a predetermined time interval just exceeding that required to extend the connection to the selector it, the relay M is caused to release to reprepare the circuit for energizing the relay l5. Thus, when the relay operates in the manner explained above, it closes its contacts 88 to complete a circuit including the battery 97 and the encircuited resistance of the rheostat 96 for energizing the cathode of the diode 95. After a short time interval required for the cathode of this tube to be heated to its electron emitting temperature, current derived irorn the source $8 is passed through the space current path of this tube to energize the lower winding of the relay 79. When both of its windings are thus energized, this relay releases and closes its contacts 83 to reprepare the circuit for energizing the relay !5. It will be understood from the above explanation that the time interval during which the relay "I9 is operated is determined entirely by the setting of the rheostat 96 to fix the period required for the cathode of the diode to be heated to its electron emitting temperature. Preferably, this rheostat is so adjusted that the indicated time interval only slightly exceeds that period which is required for a connection to be extended through a line switch to a selector. This period may be of the order of one or two seconds.

After the relay l9 releases, the relay i5 is rendered responsive to any transient or signal voltage that may be incidentally transmitted through the detector Gil. The purpose of utilizing the relay 8! to increase the negative bias voltage upon the control grid 42g of the control tube 42 in the manner explained above now becomes apparent. Thus by increasing this voltage the transient detector 46 is rendered incapable of passing the dial tone signal voltage and such incidental transient voltages as may be impressed upon the input terminals thereof. Accordingly, false operation of the relay i5 is prevented. The above-noted transient and signal voltages produced across the lineterminals of the hybrid coil 3i incident to the extension of a connection from the observation point to the selector l6, are also directly impressed across the input terminals 69 of the impedance detector nil. As previously stated, however, with the line windings of the hybrid coil 3l bridged across these terminals, the oscillator portion of the impedance detector til is prevented from operating. Accordingly, the transient and signal voltages appearing across the input terminals 69 of this detector are prevented from causing false operation of the pulsing relay 92.

When the dial tone signal is received at the substation A, the calling party may actuate the dial of the calling device 20a in accordance with the digits of the directory number designating the desired substation. For purposes of explanation it may be assumed that the substation B is desired by the calling party. When the dial of the calling device Zila is moved oil normal at the Start of each dialing operation, the lower shunt 1'3 springs of 'thi's device are closed to complet'e an obvious path for short-circuiting the transmitter Zlwand'the receiver 222; thereby: to lower the resistance of- 'the loop circuit connectingzthe sub station A with the connector I9; and toiprevent the transmitted impulses from being reproduced as clicks by-the receiver- 22a. These shunt springs are obviously opened to interrupt= the short-circuiting path. each timethe' dial-of the calling device: 28a is returned to its. normal position. During eachvreturnv movement of this dial,.. the upper pulsing-z springs.) of 1 the calling: device 2.9a areopened' andrfclosed a: number'of. timesto transmit .alcorrespondinginumb'er ofzimpulses :overr'the line 8.. These impulsesrareiollowediby the back bridge relayr291. I'm-this regardziit willibe. understood that each time the impulsing:..springs of the calling devicetlda; are opened, .a' transient voltage of rather. large... amplitude is. produced. between the .conductorsz'ofzttheelinea! 0, and the impedance of: the. connection. as." set. up between. the: substation; A andv the observation point, and; as measured across. the. lhpllti terminals" 69 of. the impedancedetector -60, is "substantially increased. Itzwill. alsotrbe understood; that: each: time. the. pulsingisprings ofzthercalling d'evicefifla. are closed atzthe: end :of-zthe'open-circuit period of i an impulse, a transient voltage of lessermagnitude is produced. across the conductors oftheline l9 and the impedance between the input terminals 69 of theimpedance detector. 601 is substantially. decreased.

The? transient. voltage produced" between the. conductors of the line: It at the beginning of the open-circuitperiod of: the first impulse of the-first'digitrdialediat the substation A, is trans mitted through the hybrid. coil 31; amplified by the channel amplifier; 35:: and impressed across the input electrodesiofthe amplifier tube M. This voltage as amplified." throughthetube 41 isv impressed between the"; input electrodes of the control tube 42 and is of. sufiicient amplitude'to. overcome the negative bias. afforded'by the bias batteries fill and 53. Thus the transient is passed to the output circuit of the tube 42-and transmit:

ted through the rectifier. 43 to causethe reopera; tion. of the relay- [5. Upon operating; the second. time, the relay "l .closesitscontacts 'liiitdagain' energize the upper winding-of therelay Tl andito complete a circuit through the contacts 83;for.

energizing the parallel-connected windings-.ofrthe two relays 81 and 89.. Therela-y 89, upon operating, locks up ina circuit whichincludesthe contacts dandthe contactsof theswitch- 94.. At its contacts-9i, the relay. 89 opens the pathover. which the linewindings of the hybrid-=coil.-3.l are bridged acrossthe conductors of theline lfl and' across the input terminals 690i theimpedance detector. 60. It will. beobserved that after this path is opened the transient or signalvoltages thereafterappearing acrossthe. conductors. of. the line i ll cannotbe. transmittedthrough the hybrid coil 31 and the amplifier 35 to the input terminals. of the. transient detector 4G. Moreover; withthis' path. opened, the. low impedance presented'ilby the linewindings of the hybrid coilfSl islno longer. connected in shunt with the input terminals 59' of'theimpedance detector 60 tov interfere with the response of this detector to changes inthe impedance of the. signaling circuit. afforded. by the. continued operation. of the pulsing springs in the calling device 20a- After the relay 8'! is operated,.it Will, byvirtue of the continued energization ofrits: lower wind ing, .remain-.operated after: its-upper. Winding;.,-. is

deenergized. This relay, in operating, closes'its" contacts 88 to complete a path through the contacts 1 flefor" short-circuiting the 1 input terminals of the West-east channel amplifier 35. The shortcircuiting of these terminals prevents transient voltages appearing across the conductors ofthe line- I i, and produced incident to the extension of the connection to the desired substation B, from being transmitted through the hybrid coil 33; the amplifier the unbalanced hybrid system including the hybrid coil 3i and the ampli-- fier 35 to*the input" terminals of the transient detector Thus this-detector is prevented from falsely responding to those transient Voltages produced on the line I l incident to the extension. ofthe connection. More generally considered, it will be'noted that following the operation of the tworelaysBT ands 'fi; the signal current repeater 38" is rendered completely inactive to transmit signal and transient voltages therethrough. in

either-direction. Thus, the transient detector 40' is rendered completely inactive and the clialton'e si'g-nalf transmittedthrough the repeater 38 in response to the first operation of the relay [5 is.

terminated.

When the impedance across the terminals 69:

is increased in the. manner just explained, op-- eration of'the oscillator sectionof the impedance detector'fifl' is initiated; The oscillatory Voltage produced: across the output circuitoi the tube 61- is transmittedrto the inputcircuit of the rectifier tube? 88: and: alternate half-cycles thereof are passed: through thelwincling oi'the pulsing relay -92: This current is smoothed through the action oftheconden'serzlfi so that th'erelay 82 is eflec tively energized'ibyja uni-directional current: Irnclosessits; contacts Ettorecomplete the loop circuit extendingrto the selector it and thus complete. the; impulse. transmitted tothis seiector. The response of theinipedance detector 6% to the other impulses transmitted to the observation point through continued operation of the calling. device ma: is exactly the same as just explained. In thisregard it will be understood that the impulses of the first digit as repeated to the selector tor to extend the con ch, for example, as the connector tBshai/ing, access to the desired. line 3. Eollowing the switch-through operation of the selector. lu-andduringtm inter-digit pauseba tWeenthefirstl-and seconddigits, the connector 58', for example, is seized andconditionedfor operation... The. impulses. ofthesecondand third digits, as; reppeated'to the connector it through operation oftlie impedance detector til and the pulsing. relay 92', are utiiizedto position the wipers of this connector on the contacts terminating the conductorsofthe line 3!. After this line is selected it is tested. in theusual manner to determine the idle. o r-busy condition thereof, and, ii? busy, the usual busy signal-is returned over the line H. Onthe-other.hand,v if the line tests idle, ringing current is transmitted; thereover tov energize :the

ringer provided at the substation B and the usual ring-back tone signal is returned over the line i i.

The attendant at the point of observation may, by retaining the key 26 in its dial and talk position, monitor the dialing operation to determine when this operation is completed. In this regard it will be understood that each impulse transmitted by the calling device 29a produces two transient Voltages across the conductors of the line Iii, which are reproduced as clicks by the receiver of the operators headset 25. Accordingly the attendant, upon observing that the clicks have stopped, is informed that the dialing operation is completed. Immediately after the dialing operation is completed, the key 14 may be actuated from its illustrated normal position to an oil-normal position wherein the contacts Ma, No and Me thereof are respectively disengaged, and the contacts Mb, 'Md and 14 are respectively engaged. At the contacts Ma, the path for bridging the input terminals 69 of the impedance detector 68 across the conductors of the line In is opened, thereby to render this detector wholly inactive. At the contacts 74?), the line winding-s of the hybrid coil 35 are again bridged across the conductors of the line Iii. At the contacts Mo and Md, the loop circuit extending from the point of observation to the connector I8 is rearranged so that the relay contacts 84 and 93 are excluded therefrom. At the contacts Me, the path shortcircuiting the input terminals of the amplifier 35 is opened, thereby to render the signal current repeater 3i] operative to transmit signal voltages from the line H to the line Hi. At the contacts M), the output terminals of 'the other channel amplifier 35 are bridged across the transmitting winding 33a of the hybrid coil 33, thereby to render the signal current repeater 38 operative to transmit signal voltages from the line It to the line H. Incident to the last-mentioned operation, the transmitting winding 33a, of the hybrid coil 33 is effectively shunted across the input terminals of the transient detector 46, thereby to render this detector substantially non-responsive to signal and transient voltages appearing across the output terminals of the amplifier 35. Following the operations just described, the connection as set up between the sub tation A and the connector i8 is fully conditioned to transmit signal and voice frequency currents in either direction. Thus the busy or ring-back tone signal voltage impressed upon the line H in the connector !8, incident to the operation of this connector to test the line 9, is returned over the line I I through the repeater 3i) and over the connection set up between the observation point and the substation A to energize the receiver 22a, whereby the calling party is informed either that the desired line is busy or that the called substation is being rung. The attendant at the observation point also receives this information, assuming that the key 26 is retained in its dial and talk position wherein the operators headset 25 is bridged across the conductors of the line H).

If the call is answered at the called substation B the control equipment of the connector l8 operates in an entirely conventional manner to arrest the ringing operation and to complete a talking circuit between the lines 9 and H and, hence, between the substations A and B. In this regard it will be understood that signal currents produced through operation of the transmitter Zlb are transmitted to the receiver 22a over a channel which includes the line 9, the connector 18, the selector it, the line switch IS, the line II,

the hybrid coil 33, the west-east amplifier 36, the hybrid coil 3!, the line H), the line switch M, the selector I'l, the connector l9, and the line 8. Signal currents produced through operation of he transmitter Zia, on the other hand, are transmitted to the receiver 22b over a circuit which includes the line 8, the connector E9, the selector ii, the line switch 14, the line II], the hybrid coil 3i, the east-west amplifier 35, the hybrid coil 33, the line H, the line switch I5, the selector I6, the connector l8, and the line 9. So long as the operators headset 25 is bridged across the conductors of the line 10, the attendant at the observation point may, of course, monitor or listen in on the conversation held between the calling party at the substation A and the called party at the substation B.

After the conversation is terminated and the connection is released at both the calling and called ends thereof, the attendant may release the switch train through which the connection is extended to the substation B by actuating the key 38 to its open-circuit position to interrupt the loop circuit as set up between the point of observation and the connector l6. When this loop circuit is opened the connector l8, the selector l3 and the line switch l'5 are restored to normal in a'inanner well understood in the art. In this regard it will be understood that if the attendant at the point of observation desires to release the connection as set up to the substation A under observation, the key 26 may be returned to normal and the key 31 operated to its open-circuit position, thereby to interrupt the loop circuit as set up between the observing equipment and the connector 59. When this loop circuit is opened the connector iii, the selector H and the line switch M are successively released in the usual manner. Incident to the release of the connector l9, the private conductor of the line 8 is disconnected from ground to render this line accessible to the other connectors of the system.

In the event the call as initiated at the substation A is abandoned at this substation before it is fully extended to the desired substation B, the signal voltage produced through operation of the oscillator section of the impedance detector is transmitted to the receiver of the operators headset 25 to inform the attendant of the abandonment of the call. Thus when the calling party restores the receiver 22a to the supporting hook of the hook switch 23a to open the loop circuit extending between the substation A and the connector l9, operation of the oscillator section of the impedance detector 63 is initiated in the manner explained above. A portion of the oscillatory voltage appearing across the tuned tank circuit of this device is transmitted through the transformer windings 64a, the contacts 74a, and the contacts of the two keys 3? and 26 to the receiver of the operators headset 25. This signal is of audio frequency and, as reproduced by the receiver of the operators headset, produces a distinctive signal which indicates that the initiated call has been abandoned. Upon receiving this signal, the operator may release the connection as extended over the line H by operating the key 38 to interrupt the loop circuit established by way of this line. The manner in which the switches occupied with the call are released when this loop circuit is broken will be clearly apparent from the above explanation.

In order to condition the observing equipment for use in handling another call after a connection has been partially or completely extended 17 from this equipment to a called substation and :then released, the key 38 isreturned to its closedcircuit position, the key M is returned to the illustrated normal position thereof, and the key 94 is momentarily operated to. its open-circuit position. In response to the latter operation the parallel-connected windings of the three relays 11,81 and .89 are deenergized, causing all three of these relays to restore. In releasing, the relay 11 opens its contacts 18 to deenergize the two relays 8| and 85 and the upper winding of; the

relay 19, thus causing the latter relay momentarily'tc reoperate. The relay 85, upon restoring, opens its contacts 86 to deenergize the cathode of the diode 95. When this cathode .has cooled for a short time interval, space current flow therethrough is arrested to deenergize the lower winding of the relay 19 causing this relay. to restore.

After the relays 15,81 and 89 have released, the

key 94 may be returned to its closed-circuit position in order to again energize the holding Wind- 'ingsof the two relays ii and 81'. Following the operations just described the observinge'quipment is fully restored to normal and is conditioned to handle another call originating at the observed substation A or at another substation which may be placed under observation in the manner explained above.

Referring now more particularly to Figs. 5, 6

and 7 of the drawings, the call observing equipment there illustrated is substantially similar in circuit arrangement to that shown in Figs. 1, 2 and 3 of the drawings and described above. Accordingly, corresponding parts of the two difierent circuit arrangements have been identified by inserted in the channel connecting the conductors 'of the line with the'input sides of the impedance detector I60 and the transient detector-H0. A balancing network comprising the condenser .224 and the resistor '223 is connected across one set of terminals of this coil in order to balance the impedance of a connection extending to the line terminals of the'coil. Facilities are also provided in the modified arrangementfortransmitting dial tone signaling current through the repeatingcoil 222 to the line It, and'thence to a substation that has been placed under observation. These facilities include a dial tone current source 2 I? having its output terminals coupled to the dial tone signaling circuit. This signaling circuit is arranged to be directly controlled by the relays l'8l and 189 of the control relay network, in a manner such that the dial tone signal is only transmitted after a connection has been extended from the observing equipment to the first selector of a switch train. The purpose of this arrangement is to provide more positive control of the dial tone signal transmission and to prevent thesigna'l voltage from producing false operation of the impedance detector [68 or the transient detector I40. A further change which has been incorporated in the apparatus shown in Figs. 5, 6 and? of the drawings pertains to the wiring of the keys l3! and I14. It will be noted that the contacts of these keys are so wired that the conductors of '18 the line l0 may be connected either to the input terminals of the repeating coil 222 or the input terminals to the hybrid coil l3l but ,not to both sets of terminals at the same time. By virtue of these changes, it is unnecessary to provide facilities for preventing the transmission of transient and signal voltages to the transient detector I40 and theimpedance detector l-Efldurin'gthe extension of a "connection from the apparatus tea called substation.

Another change which has been incorporated in the observing equipment, shown in Figs. 5, .6 and 7 of'the drawings relates to the arrangement of the transient detector 160. Briefly considered, this detector comprises a push pu'sh connected amplifier stage including the two parallel-corrnected tubes 2M and 202; a control stage, including the two parallel-conhected tubes 20.4 and 295 which are coupled through the transformer .283 to theoutput electrodes of the amplifier tubes 20 l. and 2625; and a direct current amplifier com:- prising the two parallel-connected tubes 208 and 20S; which arereslstance coupled through the resistors 2M and 2t! to the output electrodes of the tubes .204 and 2325. A leak resistor 213 shunted by a condenser H4 is included in the path connecting the cathodes and control grids of the tubes "lllliv and N35 for the purpose of limiting the extent to which the grids of these tubes may be driven positive with respect to their :respec'- tive associatedcathodes. The input electrodes .of the-first amplifier stage are coupled to the'te-rminals 228- of the repeating coil 222 through a coupling transformer 2M and a pair of coupling condensers 2 10 andZl A-battery 212 is provided for normally biasing the control grids or the-two tubes 2M and 262 substantially to the point of anode current cut ofi. Anode current for thesix tubes of the-detector M0 is supplied from a direct current source '215 which is tapped at a point 2H5 therealong sc-that the required difference of potential exists between the-cathode and control grid of each of the two direct current amplifier tubes 238 and 209. In this regard it wi1l'be noted that the normalbias on the grids of the two tubes "288 and 2 09 is determined by the oppositelyfdi 'rected current now through the balancedresistors 206 and 2M and the space current path-softl'le tubes 2 and N95. The latter tubes normally operate with zero bias on their grids so that substantial currents traverse the resistors 29 6 and H31. These currents are obviously in such directions that the resulting voltage drops-across the resistors 2 06 and 201 are negatively applied to-the'grids of the tubes 208 and 209 in opposition to the positive bias voltage provided by the section 216a of the battery 215. The circuit constants are soadjusted that'the net bias voltage as applied'te the control grid of each ofthetdbeSQ (l8 and-209 is sufiiciently negative to -n1aintai-nthe normal space current flow through the winding of the relay 1 i5 'substantia-llybelow tnevalue required to operate this relay. Current for energiz ingthe relays ill, I19, I and H39, and the cathode heaters citric-tubes 20 I 201204,- fill-5,208, 209, I58 and W5 maybe supplied Icy-anydirect current source of suitable voltage.

In considering the operation of the equipment shown in Figs. 5, 6 and 7 of the drawings, it may be assumed that'this equipment is ope'ratively assooiated with the automatic switching apparatus shown in Fig. 4 of the drawings; that calls originating at the substation A are to be observed; that the calling device l24 provided at the point or observation is utilized to direct a connection ai's-7 ,046

the point of observation after it is released at this substation. After this connection is completed, the control portion of theapparatus may be conditioned to extend a connection over the line II to a desired substation by operating the two keys I31 and I38 to their closed-circuit positions; operating the key I74 to its ofi-normal position wherein the contacts IMb, HM and I'I4e thereof are respectively engaged and the contacts I'I4a, I'I4b and IM thereof are respectively disengaged; and operating the key I94 to its closedcircuit position to prepare a locking circuit for the relay I89 and to complete an obvious circuit for energizing the holding winding of the relay I". With the key I'I4 operated to its off-normal position, the contacts IMa and IMc thereof are respectively disengaged so that the line terminals of the hybrid coil I3I are disconnected from the conductors of the line I0. Also, with the contacts IMe of this key closed, an obvious path is completed for short-circuiting the input terminals of the west-east channel amplifier I35, thereby to prevent the signal current repeate 130 from singing.

With the apparatus thus conditioned for opera- :tion, a transient voltage is produced between the conductors of the line it] when a call is subsequently initiated at the substation A. This voltage is impressed upon the input circuit of the transient detector I40 over a channel which includes the contacts of the key I 31, the contacts IMb and HM, the repeating coil windings 2220 and 222d, and the condensers 2I0 and 2H, and appears in the secondary winding of the coupling transformer 200 as an alternating voltage having a'positive half-cycle and a negative half-cycle. After amplification through the amplifier stage comprising the two parallel-connected tubes I and 202, this voltage appears as an induced voltage in the secondary winding of the coupling transformer 203. During one half-cycle of the amplified voltage the grid of one of the two tubes, such, for example, as the tube 204, is driven negative with respect to its associated cathode, therev by to cause a sharp decrease in the magnitude of the space current flow through the resistor 206 and, hence, in the voltage drop across this resistor. Thus the negative bias applied to the control grid of the direct current amplifier tube 208 is sharply decreased to produce a substantial increase in the space current flow through this tube and, hence, through the winding of the relay I15. Coincident with the current changes just described, and during the same half-cycle-of the voltage induced in the secondary winding of the transformer 263, the control grid of the tube 205 is driven positive with respect to its associated cathode, to produce an increase in the current fl ow through the coupling resistor 201. The extent to ,which the control grid of the tube 205 is driven positive with respect to its associated cathode is limited by the action of the leak resistor 2| 3 in a well known manner. Accordingly, the increase in space current flow through the tube 205 is limited not to exceed a predetermined value. It will be understood, moreover, that since the change in space current flow through this tube is manifestedat a point along the upper fiat-portion of the tube characteristic, only a small increase in current flow through this tube, and hence in the bias voltage across the coupling resistor 201, is produced. This small increase in bias voltage across the resistor 20'! is manifested 'as a decrease in the space current fiow'throug'h the tube 209 and, hence, through the winding of the relay H5. It is noted, however, that the sharp increase in current flow through the tube 208substantially exceeds the decrease in current flow through the tube 209, and that the resulting net increase in current flow through the winding of r the relay I75 is suflicient to cause the operation "of ysient voltage appearing across the conductors .switch I5.

manner.

ing across the conductors of the line II.

this relay.

During the other half-cycle of the alternating voltage induced in the secondary winding of the coupling transformer 203 as a result of the transient voltage appearing across the conductors of the line it, the potentials on the grids of the tubes 204 and 205 are controlled to produce a sharp decrease in space current flow through the tube 205 and a relative small increase in space current flow through the tube 204. As a result the space current flow through the tube 208 i is decreased slightly and that through the'tube 209 is sharply increased, The net increase in current flow through the Winding of the relay I15 is sufficient to maintain this relay in its operated position. It will thus be understood that the relay I15 is held operated during both halves of the tranof the line I0.

In operating, the relay I15 closes its contacts I16 to complete an obvious circuit for energizing the operating winding of the relay I'I'I. After the latterrelay has operated it will remain in its operated position until its holding winding is deenergizeda In operating,the relay II'I closes its contacts M8 to complete an obvious circuit for energizing the upper Winding of the relay I19 in parallel with the winding of the relay I85. The relay H9, upon operating, opens its contacts I to deenergize the relay I I5, providing the latter relay is still in its operated position. After the relay Hi5 restores, the slow-to-operate relay I closes its contacts I83 to prepare the operating circuit for the relay I89. At its contacts I06, the relay I85 completes an obvious circuit for energizing the cathode of the diode I95. I

In operating, the relay I85 also closes its contacts I84 to complete a loop circuit to the line This circuit includes the conductors of the line I i, the contacts of the key I38, the line windings of the hybrid coil I33, and the contacts I93. When this loop circuit is completed, the line switch I5 operates to extend the connection to the first idle selector to which it has access. As-

suming that the selector I6 is seized through the equipment of this selector is conditioned to respond to receive current impulses in the usual Incident to the extension of the connection through the line switch I5 to the selector I6, at least two line switching operations are performed which result in transient voltages appear- Such voltages are transmitted through the hybrid coil i33 to the input terminals of the west-east channel amplifier I36. Since the input terminals of this amplifier are short-circuited through the contacts Ilie of the key I'M, the transient Voltages are not transmitted therethrough.

During the time interval required for the abovedescribed switching operations to be performed,

the cathode of the diode I is energized from -thecllr16nt source 591 though the rheostat I96.

emitting temperature, current derived'from the source 198 is passed through the space current path of this tube to energize the relay lBI in series with the lower winding of the relay H9. When both of its windings are thus energized, the relay I79 releases and closes its contacts 189 to reprepare the circuit for energizing the relay H5. It will be understood from the above explanation that the time interval during which the relay I79 remains operated is determined entirely by the setting of the rheostat 196 to fix theperiod required for the cathode of the diode to be heated to its electron emitting temperature. Preferably, this rheostat is so adjustedthat the measured time interval only slightly exceeds 'the period required for a connection to be extended from the line H through the line switch 15 to the selector l6.

When energized through "the space current-path of the diode [95, the relaylBl closes its contacts 259 to completea circuit including thewindings of the repeating coil 222, the contacts [14b and l-l ld of the key 114, and "the contacts of the "key 131 for transmitting signal current from the source 2i? to the line 10. From this point the dial tone signal current is-transmitted over the line H) and through the tandem-connected switches M, H and 49 to energize the receiver provided at the substation A and thus signal the calling party that the dialing operation may be started. It is noted that-the constants of the:

balancing network comprising the condenser 224 and the resistor 223 are proportioned substantially to balance the impedance of the connection as set up between the observation point and the'substation understood from a brief analysis of the circuits traversed by the signal current, that the dial tone signal voltage appearing across-the terminals 226 ofthe repeating coil 222 is substantiallynegligihle. signal to the substation A is prevented from incidentally producing a voltage across the input terminals of the impedance detector I60 and the transientde'tector "I40 ofsufficient magnitude to cause theufalse operation of either of the two relays H5 and I92. When the dial tone signal is received at the "substation A,-the calling party may actuate the up in a circuit which includes the contacts 19!] and the contacts of the key I94. At its contacts the relay I89 short-circuits the secondary winding of the dial tone transformer 2'l8,'ther-eby to terminate the dial tone signal being transmitted over the line it. At its contacts 22 I ,"the relay 'l89 opens the path over which the balancing network comprising the condenser '224 and the re- 'si'stor 223 is bridgedibetween the outer terminals of the repeating coil windings 222a and 222?), thereby to prevent the oscillator section of the im-.

:pedance detector .160 from being loaded by the impedance through these two .circuit elements.

In this regard it .willbe understood that so long as :the parallel-connected elements. 223 land" 22! N'ith this in mind, it Will be Thus the transmission oithe dial tone' 22 are connected in series with the windings 1221a and 2221) across the primary winding of'the coupling transformer 5 54, the oscillator section ,of the detector lto'is too heavily loaded to oscillate, regardless of the-impedance of the connectionas set up to the substation A. After the relay 189 is energized, :the impedance across the input terminals of the coupling transformer 15A is, however, primarilydetermined by the impedance of the connection as setup between the observation point and thesubstation A. This impedance, it will be recalled, is sharply varied to correspondingly vary the loading of the oscillator section of the impedance detector I60, as the loop circuit extending between the connector I!) and the substation A is opened and closed during each impulse transmitted through operation of the calling device Zlla.

The manner in which the impedance-detector 16B responds to the impedance changes produced across the input terminals thereof incident to the operation of the calling device "2011 is exactly the same as explained above withreference to the operation of the impedance detector 60 shown in Fig. 3 of the drawings. From this explanation it will be understood that the pulsing relay 192 functions to repeat eachimpulse dialed at the substation A to the line H. The first series of impulses, i. e., those of the first digit, are utilized in the seized selector L6 to extend the connection from the line H to a connector, such, for example, as the connector =l 8 having access to the line desired by the calling party. For explanatory purposes it may again be assumed that the substation B is desired by the party using the substation A. During the inter-digit pause between the first and second digits, the selector I6 operates to switch the calling loop circuit through to the connector I8, assuming that this connector is selected during the trunk-hunting operation of the indicated selector. When thus seized, the connector-is con ditioned to respond to the impulses of the sec ond and thirddigits of the directory number designating the substation B. The impulses of these digits are utilized to position the wipers of the connector E8 on the line terminals atwhich the conductors of the line 9 are terminated. After the line is thus selected the usual testis performed in the connector 18 to determine the idle or busy condition thereof, and, if busy, .a signal to that eliect is returned through the selector i6 and the line switch l5 to-the line I]. On the other hand, if the called line 9 tests idle, ringing current is transmitted thereover to energize the ringer provided at the substation "B and the usual ring-back tone signal is transmit.- ted'back over the line H.

It will be understood that the attendant "at-the point of observation may, by retaining the dial and talk key 1% in its operated position, monitor the dialing operation to determine when it is completed. In this regard it will be understood that each impulse transmitted by the calling device 20a produces two transient voltages across the conductors of the line [0 which are reproduced as clicks by the receiver of the operator's headset I25. Accordingly, the attendant upon observing that the clicks have ceased, is mformed'that the dialing operation is completed. Immediately after the dialing operation 'is arrested, the attendant at the point of observa- :tion may actuate the key I74 to the illustrated normal position thereof for the purpose of completinga signal and voice currenttransmission circuit between the two lines 8 and 9. Incident to this operation the contacts I14b and H411 are opened to disconnect the conductors of the line ID from the input terminals of'the repeating coil 222 and thus effectively disassociate the control portion of the observing apparatus from the connection as set up between the lines 8 and 9. -At the contacts IMa and H40 the conductors of the line III are connected to the line terminals of the hybrid coil I3I, thereby to establish a twoway transmission circuit through the signal current repeater I30 for transmitting voice and signal currents between the lines I I and III in either direction. At the contacts I'Ie. the path shortcircuiting the input terminals of the west-east 15 channel amplifier I36 is opened, thereby to render this channel operative to transmit signal curring-back or busy tone voltage impressed upon the line II in the connector I8 is returned over .the line II through the repeater I30 and over .the lines I and B to energize the receiver 22a,

whereby the calling party is informed that the desired line is busy or that the called substation is being rung. Assuming that th key I26 is retained in its dial and talk position, wherein the operator's headset I25 is bridged across the conductors of the line III, the attendant at the :observation point also receives the transmitted busy or ring-back tone signal. If the call is answered at the called substation. the control equipment of the connector I3 operates in an entirely conventional manner to arrest the ringin operation and to complete a talking circuit between the lines 9 and I I and, hence, between the substations A and B. In this regard it will be understood that signal currents produced through operation of the transmitter 2Ib are transmitted to the receiver 22a over a circuit which includes the line 9, the connector I8, the selector I6, the line switch I5, the line I I, the hybrid coil I33, the amplifier I36, the hybrid .coil I3I, the line ID, the line switch I4, the se- .-lector I1, the connector I9, and the line 8. Signal currents originating at the substation A, on the other hand, are transmitted to the receiver 22b over a channel which includes the line 8,

the connector I9, the selector I1, the line switch I4, the line III, the hybrid coil I3I, the east-west amplifier I35, the hybrid coil I33, the line II, the line switch I5, the selector, I6, the connector I8, and the line 9. So long as the operators headset I25 is bridged across the conductors of the line III, the attendant at the observation point may, of course, listen in on the conversation held between the calling party at the substation A and the called party at the substation B. After the conversation is terminated and the connection is released at both the calling and called ends thereof, the attendant may release the switch train through which the connection is extended to the substation B, by actuating the key I38 to its open-circuit position to open the outgoing loop circuit. When this loop circuit is interrupted, the connector IS, the selector I6, and the line switch I are restored to normal in the usual manner.- In this regard it will be understood that if the attendant at the point of observa- 2'4 tion'desires to release the connection as set up to the substation A under observation, the keys I26 and I31 may be returned to their respective ncrmalpositions in order to interrupt the loop 5 circuit as set up between the point of observation and the connector I9. When this loop circult is open, the connector I9, the selector I1, and the line switch I4 are successively released in the usual manner. Incident to the release of the 10 connector I9, the private conductor of the line 8 is disconnected from ground to render this line accessible to the other substations of the system.

In the event the call as initiated at the substation A is abandoned at this substation before it is fully extended to the desired substation B, the signal voltage produced through operation of the oscillator section of the impedance detector I 60 is transmitted to the receiverof the operators headset I25 to inform the attendant of the abandonment of the call. Thus when the calling party restores the receiver 22a to the supporting .hook of the hook switch 23a to open the loop circuit extending between the substation A and the connector I9, operation of the oscillator sec- 25 tion of the impedance detector I60 is initiated in the manner explained above. Incident to the operation of this device, an induced alternating voltage is produced in the winding I64a of the coupling transformer I64. This voltage is transmitted to the receiver of the operators headset I25 over a path which includes the line windings 2220 and 222d of the repeating coil 222, the contacts H42) and H412 of the key I14, and the contacts of the two keys I31 and I26. When 35 reproduced by the receiver of the operator's headset I25, this signal voltage produces a distinctive signal which indicates that the initiated call has been abandoned. Upon receiving this signal, the operator may release the connection 40 as partially completed over the line II to a desired substation, by opening the key I38 to interrupt the outgoing loop circuit. The manner in which the switches occupied with the call are released when this loop circuit is broken will be vI5 clearly apparent from the foregoing explanation.

In order to condition the control portion of the observing apparatus for use in handling another call after a connection has been extended from the point of observation and then released,

the keys .I31 and I38 are operated to their respective closed-circuit positions, the key I14 is actuated to its off-normal position, and the key I94 is momentarily operated to its open-circuit position. In response to the latter operation,

the parallel-connected windings of the two relays I11-and I89 are deenergized. In releasing, the relay I11 opens its contacts I18 to deenergize the upper winding of the relay I19 and the windon ing of the relay I85. The relay I19 momentarily reoperates to open its contacts I80 incident to the deenergization of its upper winding. The relay I86, upon restoring, opens its contacts I86 to deenergize the cathode of the diode I95. After this cathode has cooled for a short time interval,

the flow of space current therethrough is arrested to deenergize the lower winding of the relay I19 and the winding of the relay I8I, causing both of these relays to restore. Following 7 the release ofthe relays I11, I8I, I85 and I'89,

,the key I94 may be returned to its closed-circuit position in order to again energize the holding winding of the relay I 11. After this operation is performed, the observing equipment is fully restored to normal and is conditioned to handle age-7,045,

252; anothenf can). originating at the. observed. substationJitor atiany' other substation which may be under: observation in the manner explained: above;

While: two different embodiments of the invention havebeen: disclosed, it will be understood that various. modifications may be made thereinawhichare within. thetrue spirit and scope of" the invention.

What. is; claimed is:

Iii-In a telephone system which includes subs scriber control-led. automatic switching apparatus for setting up connections: between the substations" thereof; an observation point, means con-.

trollabl'e from saidobservation point for selecting any desired one of said substations, means at said-observation point for controlling said switching apparatus to force completed connections involving said one substation to be set up over a route-whichincludes said observation point, and means at said. observation point for monitoring connections set: up over said route.

2; In a.- telephone system, a substation, an observation point including facilities for monitoring connections extended therethrough, and meanscontrollable from said point for preventing said'substation from receiving incoming calls and for'forcing all calls originating at said substation to be routed through said observation point to their destinations.

3: In a telephone system which includes subscriber controlled automatic switching apparatus for settingup connections between the substation thereof, an observation point, means controllable from said observation point for selectingany desired'one of saidsubstations, means at saidobservation point for controlling said switching apparatus to force all calls outgoing from said one substation to be routed to their destinations overconnections which extend serially through said observation point, and means at said observation point for monitoring'any connection extended serially therethrough.

4. In a telephone system which includes subscriber controlled automatic switching apparatus for setting up connections between the 'substa tions thereof, an observat on point, means at said observation point for controlling said apparatus to force all calls outgoing from any selected one of said substations to be routed to their destinations over connections which extend through said" observation point and to prevent the selected substation from receiving incoming calls, and means at said observation point for monitoring any connection extended therethrough,

5. In a telephone system which includes subscriber controlled automatic switching apparatus for setting up connections between the substations thereof, an observation point, means controllable from saidobservationpoint for selecting any desired one'of said substations, means includin}; said switching apparatus for forcing completed connections involving said one substation to be set up over routes which include said observation point, and means at said observation point fOr monitoring said connections set up over said routes.

6'. In a telephone system which includes subscriber controlled automatic switching apparatus for setting up connections between the substations thereof, an observation point, means controliabj le from'sai d observation point forse'lecting any desired one of said. substations, means including said switching apparatus for forcing a'll calls outgoing fromsaid one substation to fibe routed. to. their destinations over connectionsawhichv extend, serially through said observation point, and means. at said observation pointior monitoring any connection routed serially therethrough.

'7. Inv atelephone system which includes sub,-

scriber controlled automatic switching apparatus:

for setting up connections between the substations thereof, an observation point, means includ ing said. apparatus for forcing all calls outgoing from any selected one of said substations to-betv routed to their destinations over connections: which extend through said observation point and, for preventing the selected substation from re-z ceiving incoming calls, and means at said. observation point for monitoring any connection routsed therethrough.

8. In atelephone system which includesaisubstation that'is to be observed from an observation point, means controllable from. said observation; point for establishing a connection therefrom to..-

said substation, means for holding said connection after it is released at said substation, means:

controllable over said connection. from said substation for automatically extending said connec-. tion serially through said observation point to another substation, and means at said observation point for monitoring said connection as sets connection after it is released at said one substa tion, means controllable over said connection from said one substation for automatically extending said connection serially through said observation point and said automatic switching apparatus to another substation, and means at said observation point for monitoring said connections as set up serially therethrough between said two substations.

10. In a telephone system which includes sub scriber controlled automatic switching apparatus for setting up connections between the substations thereof, an observation point, means; controllable from said observation point for routing" a connection therefrom'through said automatic. switching apparatus to any desired one of said. substations, means for holding 'said connection after it is released at said one substation, means. controllable over said connection from said one substation for automatically extending said con-.--. nection serially through said observation point and said automatic switching apparatus to an.- other substation, and means at. said observation, point for monitoring said connection as set up. serially therethrough between said two substa-- tions.

11. In a. telephonesystem which includes subs, scribes: controlled automatic switching apparatus for setting up connections between the'substaw tions thereof, an observation point, means con-- trollable from said observation point for estab-. lishi-ng a connection therefrom to any desired one of said substations, means for holding said connection after it is released at. said one substation, means for transmitting a dial tone signal over said connection when a call is subse- 

